Zeolites are a commercially important class of crystalline materials. They have distinct crystal structures with ordered pore structures which are demonstrated by distinct X-ray diffraction (XRD) patterns. The crystal structure defines cavities and pores which are characteristic of the different species.
Small pore zeolites generally have up to eight-membered ring (8-MR) structures and an average pore size less than 5 Å (0.5 nm), whereas intermediate pore zeolites generally have ten-membered ring (10-MR) structures and an average pore size of about 5 Å (0.5 nm) to about 7 Å (0.7 nm). Large pore zeolites generally have at least twelve-membered ring (12-MR) structures and an average pore size greater than about 7 Å (0.7 nm).
Within the pores of the zeolites, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, disproportionation, alkylation, and transalkylation of aromatics are governed by constraints imposed by the size of the channels of the zeolite. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the pores to react; while product selectivity occurs when some of the products cannot leave the channels or do not subsequently react. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the pores. Shape selectivity can also result from configuration constraints on diffusion where the dimensions of the molecule approach that of the pore.
Zeolite SSZ-57 is a known zeolite and was first disclosed in U.S. Pat. No. 6,544,495, issued Apr. 8, 2003 to Elomari. In 2011, SSZ-57 was assigned the framework type code *SFV by the Structure Commission of the International Zeolite Association (IZA).
The structure of SSZ-57 was recently resolved by C. Baerlocher et al. (Science 333, 1134-1137 (2011)), and is currently characterized as possessing a framework type described as a disturbed ZSM-11 (MEL) structure. The disturbances are 12-ring channels that substitute for 1 out of 16 10-ring channels, in an ordered manner. Disorder by the 12-membered rings channels results in a framework with large pockets of isolated three-dimensional 10-membered ring channel systems.
It has now been found that the large pore selectivity can be enhanced by substituting the boron in the borosilicate form of the zeolite with aluminum to yield an alumino-borosilicate material wherein substantially all of its aluminum atoms located within regions of the zeolite structure which form the 12 ring channels.